The present invention relates to manhole covers, and more particularly to lift-assisted manhole covers.
Manhole covers are well known. Generally, a manhole cover is a removable plate forming the lid over the opening of a manhole to, among other things, prevent someone from falling in and to keep unauthorized persons out. Manhole covers often weigh more than 100 pounds, partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of cast iron, sometimes with infills of concrete. This makes them inexpensive and strong, but heavy. They often feature pick holes, in which a hook handle is inserted to lift them up.
Because of the weight of manhole covers, various efforts have been made to make them easier to open. More specifically, spring-assisted manhole covers are disclosed in U.S. Pat. Nos. 6,446,307 to Wilkins, 5,788,406 to Hernandez, 5,507,590 to Argandona, and U.S. Patent Application 2005/0244227 to Akkala. Each of these manhole covers swing along a vertically arced path. Unfortunately, in some circumstances, the cost of providing the necessary spring force to assist the cover in such a manner is prohibitive. Further, the manhole cover may not have room to swing a full vertical arc.
These issues have been addressed by horizontal swing spring-assisted manhole covers, such as those disclosed in U.S. Pat. No. 5,184,422 to Wade and German Patent DE19514636 to Galvanetto. In these covers, the spring applies a permanent lifting force to the cover and when the cover is unlocked the spring assists the cover vertically to just above the top edge of the manhole frame. Once the cover clears the edge of the manhole frame, the cover may be moved horizontally to allow access to the manhole shaft. Unfortunately, in Galvanetto, access to the manhole is partially blocked even when the cover is fully opened. Wade suffers in that the spring and locking mechanism extend substantially higher than the surface of the manhole cover making it difficult to drive a car or walk over the manhole cover.